Re: Subclassing NSString?

[Richard Frith-Macdonald]

On Tuesday, January 2, 2001, at 12:09 AM, Frederic wrote:

> Hi, 
>  
> I need to create NSString which are unique in memory. (I actually need 
> that for performance reasons). 
> So I decided to subclass NSString and create a class method 
> +(id)keyWithString:(NSString*)keyName which  
> looks first in a hashtable to ensure it returns a singleton. 
> But when invoking [[MyNSStringSubClass] alloc] initWithString:keyName], 
> it fails saying: 
> "subclass ASKey should override 
> initWithCharactersNoCopy:length:freeWhenDone:" 
>  
> Is this really necessary and how should I do that? 

Probably you want to add a category of NSString that implements the
+keyWithString: method, rather than subclassing.

If there is some reason that you actually need a subclass ... then,
because of the nature of class clusters, you *do* need to do more work
for your subclass, since the public NSString class is abstract ... it
doesn't actually store any string data, but depends on a few 'primitive'
methods to access the data storage provided by its concrete subclasses.

For NSString, the primitives (in GNUstep) are -

  -initWithCharactersNoCopy:length:freeWhenDone:
  -characterAtIndex:
  -length

while NSMutableString also requires -

  -initWithCapacity:
  -replaceCharactersinRange:withString:

In both cases, you will need to implement -dealloc to release the
storage your subclass uses.

In MacOS-X, it's recommended that you override *all* the initialisers for
the class cluster.  I've tried to ensure that the GNUstep implementation
of the initialisers will do something sensible as long as the two I
mentioned above are implemented in your subclass.